54. High Altitude Determines Who Survives in Tibet

Darwinian natural selection is at work among the communities living in the Tibetan mountains, according to Case Western Reserve University anthropologist Cynthia Beall. She reported at the American Association of Physical Anthropologists meeting in March that women who carry more oxygen in their blood have more than twice as many surviving children as women who carry less oxygen. “We determined that the strength of natural selection at altitude was even stronger than the strength of natural selection by falciparum malaria, and that is the classic example,” Beall said.

Women who carry at least one copy of a gene variant, or allele, that codes for high oxygen saturation had 125 percent more surviving children than those who carry two copies of a low-saturation allele. By contrast, women who carry a sickle-cell allele, which protects against malaria, have only about 50 percent more surviving children in malaria-infested regions than women lacking the variant.

Beall’s team hasn’t yet identified the gene that provides such protection, but that’s next on her agenda. In the meantime, she thinks the reason the selective difference at altitude is so large is that the altitude is constant, affecting people every day. Malaria, on the other hand, ebbs and flows over time, so the physiological benefits associated with the sickle-cell allele may be tempered.